Could this maximum on the luminosity be responsible for the non-detections of dedicated recent surveys [1]}, [2]}, [3]}? Roughly, these surveys typically reached sensitivities down to \({L_{\textnormal {H}\,\alpha }} \sim 10^{-7}\) –\(10^{-6}~{L_\odot }\)  beyond 15 au.
For most extinction geometries (see Figure REF ), this is a few to several orders of magnitude below the peak luminosity. Only for low masses and small filling factors could extinction push a planet into non-detectability, at high accretion rates (in Figure REF , where the dotted lines differ from the solid ones).
Closer in to the surveyed stars, where planets are expected to be more numerous (e.g. [4]}, [5]}, [6]}, [7]}), the sensitivity curves become quickly much less constraining; that is, the \({L_{\textnormal {H}\,\alpha }}\)  upper limits are higher. In that case, extinction affects only the higher-mass planets, but less than lower-mass planets. The low-mass planetsFor example \(M_{\mathrm {p}}\lesssim 2~M_{\mathrm {J}}\) , but the steepness of the sensitivity curves implies that no single value is representative. are not detectable in any case.
Altogether, extinction by the accreting gas does not appear to be the reason why so few accreting planets have been observed.
Other, more likely explanations are discussed in [8]} and include the intrinsic rarity of giant planets at the large separations from their host stars to which observations have been sensitive up to now. Depending on the value of the dust opacity (Figure REF ), however, the dust could also contribute to hiding accretors. Finally, in general the circumstellar disc could (also) be a source of extinction, to which we return at the end of Section REF .
